C H A P T E R 1
Introduction
T H E W E L F A RE of plants is of particular interest to those most directly concerne d with the growth of plants and the manufacture and distri- bution of plant products. It is of concern not only to farmers and work- ers in industries that process agricultural products, but also to innu- merable workers in supporting industries w h o se livelihood d e p e n ds on making e q u i p m e nt or products u s ed in processing plant products — for example, machinery for textile industries — or on distributing the raw or manufactured agricultural products. In the final analysis, how- ever, the welfare of plants should b e of concern to every one of us as consumers of plants and of the endless series of products derived from plants.
T h e growth and yield of plants d e p e n d on the availability of nu- trients and water in the soil where they grow and on the maintenance within certain ranges of such environmental factors as light, tempera- ture, and p H. Plant growth and yield d e p e n d also on protecting the plants from parasites. Anything that affects the health of plants may af- fect their growth and yield and may seriously reduce their usefulness to themselves and to mankind. Plant pathogens, unfavorable weather,
I
and insect pests are the most common causes of reduction or destruc- tion of plant growth and production. Plants suffer from diseases w h o se causes are similar to those affecting animals and man. Although there is no evidence that plants feel pain and discomfort, the d e v e l o p m e nt of disease follows the same steps and is usually as complex in plants as it is in animals and man.
Plant pathology is the study of (1) the living entities and the envi- ronmental conditions that cause d i s e a se in plants; (2) the mechanisms by which these factors produce d i s e a se in plants; (3) the interactions b e t w e en the disease-causing agents and the d i s e a s ed plant; and (4) the methods of preventing disease, alleviating the d a m a ge it causes, or controlling a d i s e a se either before or after it develops in a plant.
Plant pathologists study the diseases c a u s ed by fungi, bacteria, par- asitic higher plants, viruses, and nematodes. T h e y also study plant disorders c a u s ed by the excess, imbalance, or lack of certain physical or chemical factors, such as moisture, temperature, and nutrients.
Plant damages c a u s ed by insects, man, or other animals are not ordi- narily included in the study of plant pathology.
Plant pathology may call upon the basic techniques and k n o w l e d ge of botany, mycology, bacteriology, virology, nematology, plant anato- my, plant physiology, genetics, biochemistry, horticulture, soil sci- ence , forestry, chemistry, physics, meteorology, and many other branches of science. Plant pathology profits from advances in any one of these sciences, and many advances in other sciences have b e e n m a de in the attempt to solve phytopathological problems. A good knowledge of at least the basic facts of the related sciences is indis- p e n s a b le for efficient performance by any plant pathologist.
Although plant pathology as a science attempts to increase our knowledge of the causes and the d e v e l o p m e nt of plant diseases, it is also a science with a more practical goal. T h e purpose is to d e v e l op controls for all plant diseases. T h e goal is to save the produce which today is destroyed by plant diseases and to make it available to the growers who toil to produce it and to the hungry and ill-clothed mil- lions of our increasingly overpopulated world.
T h e Concept of D i s e a se in Plants
A plant is healthy or normal when it can carry out its physiological functions to the best of its genetic potential. T h e se functions include normal cell division, differentiation, and development; absorption of water and minerals from the soil and translocation of these throughout
2 1. INTRODUCTION
The Concept of Diseases in Plants 3 the plant; photosynthesis and translocation of the photosynthetic products to areas of utilization or storage; metabolism of synthesized c o m p o u n d s; reproduction; and storage of food supplies for overwin- tering or reproduction.
Wheneve r plants are disturbed by pathogens or by certain environ- mental conditions and one or more of these functions are interfered with b e y o nd a certain deviation from the normal, then the plants be- come diseased. T h e primary causes of d i s e a se are either pathogens or factors in the physical environment, but the specific mechanisms by which diseases are produced vary considerably with the causal agent and sometimes with the plant. At first the reaction of the plant to the disease-causing agent is at the site of affliction, is of a chemical nature, and is invisible. Soon, however, the reaction b e c o m es more wide- spread and histological changes take place that manifest themselves macroscopically and constitute the symptoms of the disease.
Affected cells and tissues of d i s e a s ed plants are usually w e a k e n ed or destroyed by the disease-causing agents. T h e ability of such cells and tissues to perform their normal physiological functions is r e d u c ed or completely eliminated; as a result, plant growth is r e d u c ed or the plant dies. T h e kinds of cells and tissues that b e c o me infected deter- mine the type of physiological function of the plant that will b e inter- fered with first. T h u s, infection of the root (e.g., root rots) interferes with absorption of water and nutrients from the soil; infection of the xylem vessels (vascular wilts, certain cankers) interferes with translo- cation of water and minerals to the crown of the plant; infection of the foliage (leaf spots, blights) interferes with photosynthesis; infection of the cortex (cortical canker, viral infections of phloem) interferes with the downward translocation of photosynthetic products; flower infec- tions (bacterial blights, viral and fungal infections of flowers) interfere with reproduction; and infections of fruit (fruit rots) interfere with reproduction and/or storage of reserve foods for the ne w plant (Fig. 1).
In contrast to the above, there is another group of diseases in which the affected cells, instead of b e i ng w e a k e n ed or destroyed, are stimu- lated to divide much faster (hyperplasia) or to enlarge a great deal more (hypertrophy) than normal cells. Such hyperplastic or hypertro- p h i ed cells result in the d e v e l o p m e nt of usually nonfunctioning, ab- normally large, or abnormally proliferating organs or in the production of amorphous overgrowths on normal-looking organs. Overstimulated cells and tissues not only divert much of the available food stuffs to themselves and away from the normal tissues, but frequently, by their excessive growth, crush adjacent normal tissues and interfere with the physiological functions of the plant.
Proteins synthesized Vitamins and
hormones formed
Reproduction and storage of starch, proteins,and fats
Shoot blight
j / ^ . L e af b l i g ht
Fruit spot
Photosynthesis (Food manufacture)
Sugars and nitrogen form amino acids
Uptake of water
///and minerals
vyProtein synthesized
F i g. 1. Schematic representation of the basic functions in a plant and of the interfer
ence with these functions c a u s e d by s o m e common types of plant diseases.
Fruit rot
Leaf spot
Canker r translocation
^ \v Λ of water s / ) a n c
l
i!I /) mineralsΜ / Food \, η translocation
Vascular wilt Crown gall
< jtoot rot Transpiration
4 4 /
Lighl Carbon dioxide .
I l l
Importance of Plant Diseases—Losses 5 D i s e a se in plants, then, can b e defined as any disturbance brought about by a living entity or an environmental factor which interferes with manufacture, translocation, or utilization of food, mineral nu- trients, and water in such a way that the affected plant changes in ap- pearance and/or yields less than a normal, healthy plant of the s a me variety. Pathogens may cause d i s e a se in plants by (1) consuming the contents of the host cells upon contact; (2) killing, or disturbing the metabolism of host cells through toxins, enzymes, or growth-regulat- ing substances they secrete, (3) w e a k e n i ng the host by continually absorbing food from the host cells for their own use, and (4) blocking the transportation of food, mineral nutrients, and water through the conductive tissues. D i s e a s es c a u s ed by environmental factors result from extremes in the conditions supporting life (temperature, light, etc.) and in amounts of chemicals absorbed or required by plants.
T h e expression of d i s e a se is the s um of biochemical reactions taking place at the point of contact of the pathogen with the host cells or at the points of reaction of the enzymes, toxins, etc., secreted by the pathogen, with the host cells. A whole plant, however, may b e c o me d i s e a s ed when only a small area involving the main conductive tis- sues of the plant is invaded by certain pathogens and translocation failure results. Although man recognizes plant diseases by the symp- toms or the unhealthy condition of affected plants, it should b e re- m e m b e r e d that diseases are not symptoms or conditions, nor are they the instigators of the symptoms or conditions, but rather they are the interactions b e t w e en host and pathogen or b e t w e en host and adverse environmental factors that result in such symptoms or abnormal con- ditions. T h u s, the pathogen itself is not the d i s e a se —it is the cause of the disease. D i s e a se is the sum of the normal chemical reactions that are inhibited and of the abnormal chemical reactions induced inside the cells and in the tissues of the plant as a result of the irritation brought about by the causal agent.
Importance of Plant Diseases —Losses
Plant diseases are important to man b e c a u se they cause d a m a ge to plants and plant products. For millions of p e o p le all over the world who still d e p e n d on their own plant produce for their existence, plant diseases can make the difference b e t w e en a happy life and a life haunted by hunger or can even result in death from starvation. T h e death from starvation of a quarter million Irish in 1845 and much of the hunger of the underfed millions living in the u n d e r d e v e l o p e d,
6 1. INTRODUCTION
rural countries today are morbid examples of the c o n s e q u e n c es of plant diseases. For countries where food is plentiful, plant diseases are important b e c a u se they cause economic losses to growers, they result in increased prices of products to consumers, and they destroy the beauty of the environment by damaging plants around homes, along streets, in parks, and in forests.
Plant diseases may limit the kinds of plants that can grow in an area or in an entire country by destroying all plants of certain species that are extremely susceptible to a particular d i s e a s e, as exemplified by the American chestnut, which was annihilated in North America as a timber tree by the chestnut blight disease. Plant diseases may also determine the kinds of agricultural industries and the level of employ- ment in an area by affecting the amount and kind of produce available for canning or processing by the industries in the area. On the other hand, plant diseases are responsible also for the creation of ne w in- dustries which develop chemicals, machinery, and methods to control plant diseases; the annual expenditures to this e n d amount to billions of dollars in the United States alone.
T h e kinds and amounts of losses c a u s ed by plant diseases vary with the plant or plant product, the pathogen, locality, environment, con- trol measures practiced, etc., or combinations of these factors, and may range from slight loss to 1 0 0 % loss. Plants or plant products may b e reduced in quantity by disease in the field, as i n d e ed is the case with most plant diseases, or by disease during storage, as is the case of the rots of stored fruits, vegetables, grains, and fibers. Frequently, severe losses are c a u s ed by reduction in the quality of plant products. For instance, spots, scabs, blemishes, and blotches, on fruit, vegetables, or ornamental plants may have little effect on the quantity produced, but the inferior quality of the product may reduce the market value so much that production is unprofitable or a total loss. S o me diseases, e.g., ergot of rye, make plant products unfit for human or animal con- sumption by making them poisonous.
Financial losses resulting from plant diseases may b e incurred indi- rectly by the farmer's having to plant varieties or species of plants that are resistant to d i s e a se but are less productive, or more costly, or commercially less profitable; by his having to spray or otherwise con- trol a disease, thus incurring expenses for chemicals, machinery, stor- age space, and labor; by his having to provide refrigerated warehouses and transportation vehicles, thereby increasing e x p e n s e s; by limiting the time during which products can b e kept fresh and healthy, thus forcing growers to sell during a short period when products are abun-
Classification of Plant Diseases 7
dant and prices are low; by necessitating the sorting of healthy from d i s e a s ed products, and thus increasing costs of handling plant prod- ucts.
S o me plant diseases can b e controlled almost entirely by one or another method, thus resulting in financial losses only to the amount of the cost of the control. Sometimes, however, this cost may be almost as high as, or even higher than, the return expected from the crop, as in the case of some diseases of small grains. For other diseases no effective control measures are known as yet, and only a combination of cultural practices and somewhat resistant varieties make it possible to raise a crop. For most plant diseases, however, practical controls are available although some losses may b e incurred in spite of the control measures taken. In these cases, though, the benefits from the control applied are generally much greater than the c o m b i n ed direct losses from the d i s e a se and the indirect losses d ue to expenses for control.
Classification of Plant Diseases
T h e r e are tens of thousands of diseases that affect cultivated plants.
On the average, each kind of crop plant can be affected by one hun- dred or more plant diseases. E a ch kind of pathogen may affect any- where from one variety to several dozen or e v en hundreds of species of plants. T o facilitate the study of plant diseases, they must b e grouped in s o me orderly fashion. This is necessary also for the identi- fication and s u b s e q u e nt control of any given plant disease. Any one of several criteria may b e u s ed as a basis for classification of plant dis- eases. Plant diseases are sometimes classified according to symptoms they cause (root rots, cankers, wilts, leaf spots, scabs, blights, anthrac- noses, rusts, smuts), according to the plant organ they affect (root dis- eases, stem diseases, foliage diseases, fruit diseases), or according to the types of plants affected (field crops diseases, vegetable diseases, fruit tree diseases, forest diseases, turf diseases, diseases of ornamen- tal plants). However, the most commonly u s ed criterion is the type of pathogen that causes the d i s e a se (Fig. 2). On this basis plant diseases are classified as follows:
I. Infectious Plant D i s e a s es 1. D i s e a s es c a u s ed by fungi 2. D i s e a s es c a u s ed by bacteria
3. D i s e a s es c a u s ed by parasitic higher plants 4. D i s e a s es c a u s ed by v i r u s es
5. D i s e a s es c a u s ed by n e m a t o d es
F i g. 2. Schematic diagram of the s h a p e s and sizes of certain plant pathogens in rela- tion to a plant cell.
History of Plant Pathology 9
II. Noninfectious or Physiological D i s o r d e rs 1. Nutrient deficiencies
2. Mineral toxicities
3. L a ck or e x c e ss of soil moisture 4. T oo low or too h i gh t e m p e r a t u re 5. L a ck or e x c e ss of light
6. L a ck of oxygen 7. Air pollution
8. Soil acidity or alkalinity ( p H)
History of Plant Pathology
Man b e c a me painfully aware of plant diseases in the early times of antiquity. This is e v i d e n c ed by the inclusion in the O ld Testament of blasting and mildew, along with human diseases and war, a m o ng the great scourges of mankind. T h e Greek philosopher Theophrastus (370-286 B.C.) was the first actually to study and write about diseases of trees, cereals, and l e g u m e s, although his approach was observa- tional and speculative rather than experimental. H e noticed that dif- ferent kinds of plants are more liable to certain diseases than other plants, that even varieties may differ in their susceptibility to a partic- ular disease, and that the soil and weather may influence the severity of a disease. H e b e l i e v ed that the diseases were produced sponta- neously by the plant, but h e also s u g g e s t ed that, rarely, diseases may b e c a u s ed by creatures that do not c o me from the plant itself, but from without. E v en such creatures, however, w e re thought to have arisen spontaneously in or on d e c o m p o s i ng matter. During the following 2000 years, little was a d d ed to the knowledge of plant pathology, al- though reference s to the ravages of plant diseases a p p e a r ed in the writings of several contemporary historians.
T h e discovery of the c o m p o u nd microscope around the m i d d le of the 17th century o p e n ed a ne w era in the life sciences. T h e anatomy of plants was studied and described, and in 1675 L e e u w e n h o e k, work- ing with a microscope h e had built, discovered bacteria and many other microorganisms. In 1729, Micheli u s ed the microscope to study many fungi and identified their " s e e d s" (spores). In a series of experi- ments h e s h o w ed that when " s e e d s" of each kind of fungus studied were scattered on freshly cut surfaces of melons, quince, and pears, they consistently produced crops of their own kind. H e also pointed out that a few aberrant fungal growths on s e e d ed or uninoculated con- trol surfaces w e re the result of airborne spores that h a p p e n ed to fall there.
1. INTRODUCTION
In 1755, Tillet presented the results of well-replicated and con- trolled plot experiments in which h e a d d ed the black dust from bunted wheat to s e ed from healthy wheat and observed that bunt was much more prevalent in plants p r o d u c ed from such s e ed than from nondusted seed. H e thus s h o w ed that bunt, or stinking smut, of wheat is a contagious plant disease. H e also s h o w ed that its occurrence can b e reduced by s e ed treatments. Tillet, however, b e l i e v ed that it was a poisonous substance contained in the dust, rather than living microor- ganisms, that c a u s ed the disease.
Persoon (1801) and Fries (1821) p u b l i s h ed papers on the taxonomy of fungi, but both b e l i e v ed that the rust and the smut fungi were prod- ucts of the d i s e a s ed plants rather than distinct microorganisms.
In 1807, Prevost proved conclusively that bunt is c a u s ed by a fun- gus; h e studied the spores, their production and germination. H e could control the d i s e a se by d i p p i ng the s e ed in a copper sulfate solu- tion, and h e pointed out the importance of the environment in induc- tion and d e v e l o p m e nt of the disease. Prevost's findings, however, were ahead of his time and were rejected by almost all his contempo- raries, who b e l i e v ed in spontaneous generation.
T h e devastating epidemics of late blight of potato in Northern Eu- rope, particularly Ireland, in the 1840's tragically dramatized the im- portance of plant diseases and greatly stimulated interest in their causes. T h e destruction of the potato crop in Ireland in 1845 and 1846 c a u s ed w i d e s p r e ad famine which resulted in the death of hundreds of thousands of p e o p le and the immigration of more than one and a half million Irish to the United States. Several investigators described var- ious aspects of the d i s e a se and of the pathogen, but it was Speer- schneider (1857) and D e B a ry (1861, 1863) who finally proved experi- mentally that the fungus Phytophthora infestans is the cause of the disease.
D e B a ry (1853), working at first with smut and rust fungi, established conclusively that fungi are causes, not results, of plant disease. H e described the microscopical structure and d e v e l o p m e nt of many smut and rust fungi and the relationships of these fungi to the tissues of the d i s e a s ed plants. D e B a ry also m a de great contributions with his stud- ies of the Peronosporaceae and the diseases they incite (downy mildews), especially the late blight of potato, his discovery of the oc- currence of two alternate hosts in the rusts, and his studies of the physiology of the Sclerotinia rot diseases of carrots and other vegeta- bles. In the Sclerotinia diseases, D e B a ry noted that host cells were killed in advance of the invading hyphae of the fungus and that juice from rotted tissue could break down healthy host tissue. B o i l ed juice 10
History of Plant Pathology
from rotted tissue had no effect on healthy tissue. D e B a ry concluded that the pathogen produces enzymes that d e g r a de and kill plant cells from which the fungus can then obtain its nutrients.
Berkeley (1845, 1857) and Kuhn (1858) p u b l i s h ed numerous papers on diseases of cereals, vegetables, and other plants. T h e latter pub- lished the first textbook of plant pathology, in which climatic and soil conditions, insects, parasitic higher plants, and microorganisms were listed as causes of plant diseases.
Brefeld (1875, 1883,1912 ) contributed greatly to plant pathology by introducing and developing modern techniques for growing microor- ganisms in pure culture. In this h e was assisted a great deal by the methods and refinements d e v e l o p ed by Koch, Petri, and others. Bre- feld also studied and illustrated the complete life cycles of the smut fungi and diseases of cereal crops.
In 1878, a n e w d i s e a s e, the downy m i l d ew of grape, a p p e a r ed in E u r o p e. T h e d i s e a se had b e e n introduced from the United States, spread rapidly, and threatened to ruin the vineyards of E u r o p e. In 1882, Millardet noticed that vines which had b e e n sprayed with the bluish-white mixture of copper sulfate and lime to deter pilferers re- tained their leaves through the season, whereas the leaves of un- treated vines had b e e n killed by the d i s e a se and had fallen to the ground. After numerous spraying experiments with various combina- tions of copper, calcium, and iron salts, Millardet finally concluded in 1885 that a mixture of copper sulfate and hydrated lime could effec- tively control the downy m i l d ew of grape. This mixture b e c a me known as " B o r d e a ux M i x t u r e /' and its success in controlling downy mildews and many other foliage d i s e a s es was spectacular. E v en today Bordeaux mixture is the most widely u s ed fungicide all over the world. T h e discovery of Bordeaux mixture gave great encouragement and stimulus to the study of the nature and control of plant diseases.
In the early 1900's, studies of the genetics of d i s e a se resistance in the cereal rusts by Biffen (1905, 1912), and in the Fusarium wilts of cotton, watermelon, and c o w p ea by Orton (1900, 1909), led to the selection and b r e e d i ng of resistant varieties in these and in other crops.
In the meantime, Pasteur and Koch had proved in 1876 that the animal d i s e a se anthrax is incited by a bacterium. In 1878, Burrill s h o w ed that fire blight of pear and a p p le is also c a u s ed by a bacterium.
Soon after that several other plant diseases w e re shown to b e c a u s ed by bacteria; these included the yellow d i s e a se of hyacinth (Wakker, 1883), the olive knot d i s e a se (Savastano, 1887), and the bacterial wilt of cucurbits (E. F. Smith, 1895). Smith's numerous and excellent con-
11
1. INTRODUCTION
tributions in s u b s e q u e nt years to the study of bacterial diseases of plants established b e y o nd any doubt the importance of bacteria as phytopathogens.
T h e first plant parasitic nematodes w e re reported by N e e d h am in 1743 within wheat galls (kernels). In 1855, Berkeley observed root- knot nematodes in root galls of cucumber. Kuhn in 1857 described the bulb and stem nematode from malformed floral heads of fuller's tassel, and in 1859 Schacht reported the sugar b e e t cyst nematodes from sugar b e e t roots. A series of studies on plant parasitic nematodes were published by C o bb from 1913 to 1932 and contributed greatly to ne- matode taxonomy, morphology, and methodology.
In 1886, Mayer was working with a serious d i s e a se of tobacco which h e n a m ed "tobacco m o s a i c ." H e could reproduce the disease by in- jecting juice from infected tobacco plants into healthy plants. T h e juice of d i s e a s ed plants remained infective even after continual heat-
ing at 60°C, although heating at 6 5 - 7 0 °C r e d u c ed its infectivity some- what and the juice lost its infectivity after several hours of heating at 80°C. Mayer also noted that the juice lost its infectivity after clarifica- tion and precipitation with a weak alcohol solution. Since no fungi were present on the d i s e a s ed plant or the filtered juice, h e concluded that tobacco mosaic was probably c a u s ed by a bacterium.
In 1891, E. F. Smith showed that the peach yellows disease was contagious, had a long incubation period, and was b ud transmitted, but h e could not determine its cause although h e suggested that it was similar to that of tobacco mosaic.
In 1892, Ivanowski showed that the causal agent of tobacco mosaic could even go through a C h a m b e r l a nd filter that retains bacteria. This led him to believe that the d i s e a se was c a u s ed by a toxin secreted by bacteria or by small bacteria that p a s s ed through the pores of the filter.
Beijerinck (1898) was the first to conclude that tobacco mosaic was caused not by a microorganism but by a contagium vivum fluidum, which he also called a virus. Beijerinck also found that the virus would infect and invade young tissue more rapidly than mature tissue, that it m o v ed in the phloem and xylem, that it reproduced itself in the living plant, and that it could survive in dried leaves and in the soil.
It was not until 1935, however, that the first major contribution was made regarding the nature of viruses. That year Stanley obtained a crystalline protein by treating juice from infected tobacco plants with ammonium sulfate. H e could reproduce the d i s e a se by inoculating healthy tobacco plants with that protein and concluded that the virus could be considered as an autocatalytic protein which could multiply 12
Identification of Plant Diseases
within living cells. In 1936, B a w d en and his colleagues demonstrated that the crystalline preparations of the virus actually consisted of pro- tein and nucleic acid. T h e first virus particles were v i e w ed with the electron microscope by Kausche and his colleagues in 1939. In 1956, Gierer and Schramm showed that the protein could b e removed from the virus and that the nucleic acid alone could infect a plant and could reproduce the complete virus.
During the 20th century, plant pathology has matured as a science.
Thousands of diseases have b e e n described, pathogens have b e e n identified, and control measures have b e e n d e v e l o p e d. T h e studies of genetics and of the physiology of d i s e a s es have b e e n e x p a n d ed great- ly, and ne w chemical compounds are b e i ng d e v e l o p ed continually to combat plant diseases. Still, this is probably just the beginning of plant pathology and of the h o pe that it holds for the future. T h e h u ge losses in plants and plant products that occur annually are the single best reminder of how much is yet to b e learned about plant diseases and their control. T h e r e are thousands of plant diseases that we know little or nothing about; there are probably ne w types of pathogens that cause plant diseases and are awaiting discovery; our knowledge of the physiology of plant diseases is dreadfully incomplete; and there must surely b e better materials and methods for controlling plant diseases that are waiting to b e produced and developed. Progress in any and all of these areas is the goal of plant pathology. And a hungry, overpopu- lated world is anxiously awaiting the results.
Identification of Plant Diseases
In identification of any plant d i s e a se the first step is to determine whether the d i s e a se is c a u s ed by an infectious agent or an environ- mental factor. Infectious agents usually produce characteristic symp- toms on some parts of the plant that reveal the presence and some- times the kind of the infectious agent. S o me infectious agents, however, especially viruses, may produce general systemic symptoms very similar to those c a u s ed by certain environmental factors.
If no pathogen can b e found on or in a d i s e a s ed plant and if the symptoms present are not typical virus symptoms or cannot be consid- ere d as b e i ng c a u s ed by ectoparasitic nematodes, environmental fac- tors may b e considered as the c a u se of the disease. T o identify the environmental factor one would look for characteristic symptoms. If no such symptoms are present, then the environment would b e exam- ined for unusual conditions (e.g., low temperature, flooding) which
13
1. INTRODUCTION
may have existed or for unsound cultural practices. In some cases spe- cial tests may b e required to determine the p H of the soil, the pres- enc e of phytotoxic minerals in high concentrations, etc.
Whe n a pathogen is found on a plant, the pathogen is identified by referenc e to special manuals; and if the pathogen is known to cause such a disease, then the identification may b e considered completed.
If, however, the pathogen found s e e ms to be the cause of the disease, but no previous reports exist to support this, then the following steps are taken to verify the hypothesis that the isolated pathogen is the cause of the disease:
1. T h e pathogen must b e found associated with the d i s e a se in all the d i s e a s ed plants examined.
2. T h e pathogen must b e isolated and grown in pure culture on nu- trient media, and its characteristics described (nonobligate parasites), or on a susceptible host plant (obligate parasites), and its appearance and effects recorded.
3. T h e pathogen from pure culture must b e inoculated on healthy plants of the s a me species or variety on which the d i s e a se appears, and it must produce the same d i s e a se on the inoculated plants.
4. T h e pathogen must b e isolated in pure culture again and its char- acteristics must b e exactly like those observed in step 2.
Whe n all the above steps (usually known as Koch's postulates) have b e e n followed and proved true, then the isolated pathogen is identi- fied as the organism accountable for the disease.
T h e steps for identification of virus diseases deviate from those listed above owing to the distinct nature and properties of viruses.
Whe n a plant is suspected to b e infected with a virus, proof that a virus causes the d i s e a se can b e obtained by transmitting the virus into healthy plants of the s a me species or variety and reproducing the symptoms of the d i s e a se in the inoculated plant. T h e identification of the virus, however, is m a de by inoculating selected differential host plants which serve as virus indicators and recording the symptoms produced on the indicators. T h e identity of the unknown virus may b e determined by comparison of its symptoms on the indicators with those produced on the s a me indicators by known viruses. More re- cently it has b e e n possible to identify viruses by serological tests and other methods.
14
